progesterone fluctuation

Hi,
  I am 8 week pregnant. In conceiving I did not had any problem at all. once my pregnancy was confirmed, my doctor was having weekly blood test for progesterone and HCG level. In my 7th week the progesterone level went down from 20.1% to 17.1%. My doctor want me to use the Prochieve  8% gel. they also did my 1st ultrasound. the baby is fine and his heartbeat is 170 which is excellent doctor says. I read on the gel that it has its own side effect ( can effect the baby development as well).
I am wondering whether the fluctuation is normal. Can I wait to see if progesterone level goes up by itself, or I must start using the gel. I don't want to have any side-effect of the gel on me and my baby. please advice me.